Cold and hollow electrode



June 28, 1960 P. LEMAlGRE-VOREAUX 2,943,226

COLD AND HOLLOW ELECTRODE Filed Jan. 31, 1956 INVENTOR PIERRE LEMAIGREVOREAUX av 6 4, 9 m M.

ATTORNEYS United States Patent O COLD AND HOLLOW ELECTRODE PierreLemaigre-Voreaux, Paris, France, assignor to S- ciete Anonyme pour lesApplications de lElectricite et des Gaz Rares-Etablissements Claude-Paz& Silva, Paris, France Filed Jan. 31', 1956, Ser. No. 562,565

2 Claims. (Cl. 313-352 This invention relates to cold and hollowelectrodes for electric discharge devices containing gas and mercuryvapour. It relates more particularly to an electrode to the inner wallof which has been fixed at least one small piece comprising, mainly orsolely, one or more rare earth metals in a metallic condition, forinstance lanthanum.

This electrode is characterized by the fact that the area of the piece,or the total area of the pieces, is small, and remains small, 'ascompared with the area of the inner wall of the electrode, preferablyless than one tenth of this wall area.

Preferably, the formation treatment to which the electrode has beensubjected and the conditions of its normal operation must be such as toavoid substantially any evaporation of the rare-earth metal or metalsdue to too high a temperature, as well, as any sputtering of this metalby ionic bombardment, since evaporation and sputtering enlarge greatlythe surface of the piece or pieces: to the original surface would beadded a wide film made of a portion of the rare-earth metal, depositedon the inner wall of the electrode and on the envelope of the dischargedevice provided with an electrode submitted to such treatment orvconditions. rare-earth metal in contact with the discharge atmospherewould very substantially shorten the life of the discharge device, as itwill be explained later.

Cold electrodes generally consist of a hollow metal cylinder which isopen at one end and secured to one or more current lead-in wires, theedge of the aperture at the open end being, generally, provided with aninsulating piece which protects said edge from ionic bombardment andprevents the electrode from touching the wall of the discharge tube.These electrodes have long lives but they cause an important voltagedrop. With alternating current the voltage drop for the two electrodes,that is the sum of the cathode and anode drops, is of the order of 200volts, R.M.S. This voltage drop may be decreased by activating theelectrode, that is by providing its inner wall with an electron-emittingcoating, ca-

pable of emitting electrons readily at the relatively low operatingtemperature of the electrode, that is a temperature which is of theorder of 150 C. This coating,

which however, generally consists of alkaline-earth oxides, loses itsefficiency at the end of a time of operation less than the averagelife-time of a nonactivated cold electrode, and often causes theappearance of stains in tubes provided with such electrodes.

One object of the invention is to provide electrodes the voltage drop ofwhich is substantially smaller than that of cold electrodes, nonactivated, this voltage dropv remaining low for a very. long duration.Other advantages of the electrodes according to'the invention will bepointed out hereinafter.

One embodiment of the invention will be described hereinafter by Way ofexample, with reference to the accompanying drawing inwhich:

Figure 1 shows an electrode according to the inventiorij Figure 2 showsthe variation, as a function of the dis- A broad surface of chargecurrent, of the voltage drop at the two electrodes of a dischargeapparatus provided either with electrodes according to the invention orelectrodes which are similar except that they comprise no rare earthmetal or alloy.

Figure 1 shows, in longitudinal section, an electrode according to theinvention before it is mounted inside the envelope of a dischargedevice.

This electrode comprises a cylinder 1 made of sheet metal, for instancenickel plated iron sheet, at the ends of which are two steatite parts 2,7. The part 2 is provided with an orifice 3 allowing the electricdischarge an access to the inside of the electrode. The part 2 is alsoformed with a flange 5 which prevents the discharge from occuring on theedge 4 of the cylinder 1 and from causing a heavy sputtering of thesheet metal at that place.

The other steatite part 7 closes the end of the cylinder 1 opposite thatthroughwhich the discharge passes and this closure need not beair-tight.

A wire 8, welded to cylinder 1 supports the electrode and supplies itwith electric current.

Some pieces 6 of metal or of an alloy of rare earth metals, are securedon the inner wall of the cylinder 1. In the embodiment illustrated inFigure 1, two pieces 6 are shown and they comprise segments of lanthanumwire, welded at their mid points to cylinder 1. It is not necessary forthe lanthanumto be extremely pure, it being suflicient that thoseimpurities which may be detrimental such 'as oxides or nitrides, do notexist in troublesome' amounts. I

Lanthanum may be replaced by cerium or by an alloy 7 of rare earthmetals. Good results were obtained, for;

instance, with an alloy according to the following specifi-' Theelectrode, once it'has been provided with' the lanthanum pieces 6 andits current lead-in wireS or wires if it is provided with severalcurrent lead-in wires,

is mounted inside a segment of glass tube terminating in a bottom intowhich latter the current lead-in wire or wires is or are sealedvacuum-tightly. A tube segment thus provided with an electrode is thenwelded at each end of a glass tube which may possibly be ternally with alayer of fluorescent material.

The discharge tube thus obtained is then subjected to, a number oftreatments which form its electrodes and, which put it into anoperatingcondition; It is possible,.

for example, after having introduced a drop of mercury intothe tube, tode-gas the latter, together with its elec trodes, byconnecting said tubethrough its exhausttube with a vacuum pump and passing a'dischargetherethrough, with an intensity substantially higher than that of thenormal tube operation. -When the electrodes have been. heated for asufi-icient time to a bright red and the glass; of the tube is still ata suitable temperaturqthe discharge. is stopped and pumping is continuednntila good vacuum. 7 is obtained. The presence of traces of'oxygen andof;

nitrogen during these treatments restrain very materially theevaporation and the sputtering of the rare earth metals, especiallyduring the electrode bombardment. Evaporationand sputtering may befur-flier lessened bya shortening of, the treatments duration. The tubeand ,its 'elec-J trodes may also be heated during this de-gassing,notgby;

means of a heavy discharge, but by placing the tube, concoated in-'taining no mercury, in an oven, while heating its electrodes by a highfrequency field. These two methods of operation are usual in themanufacturing of cold cathode tubes.

- With the electrodes according to the invention; it is not necessary.to carry the .de-gassing by heating as far, and a more even heating,with no discharge or high frequency may be sutlicient. The residualpressureat the time when the pumping is stopped, does not have to be aslow as when usual cold electrodes are used, since the rare earth metalpieces 6 will absorb the small amount of nonrare gases which has notbeen eliminated by the de-gassing. This constitutes an additionaladvantage of the electrodes according to the invention.

As is well known, once the de-gassing has been completed, the tube isfilled with a permanent gas with a pressure of a few millimeters ofmercury. This gas, may for example comprise argon, technically pure orcontaining a little nitrogen,.or alternatively it may comprise a mixtureof argon and neon. After this filling, the tube is separated from thepumping device, the mercury is introduced if this had not been donepreviously, the exhaust tube is closed and cut and then the tube isoperated for some time to bring about the diffusion of part of themercury.

The heating of the electrode during its de-gassing may cause, at leastpartly, the melting of the pieces 6 of lanthanum or of other rare earthmetal or alloy. This melting may cause these pieces to spread somewhatbut the material of these pieces must not spread upon a large portion ofthe inner area of the electrode.

It must be avoided that rare earth metal leave pieces 6 in perceptibleamount and form a deposit on the envelope of the lamp and on the surfaceof theelectrode, for instance because the electrode has been heated to atemperature at which said metal evaporates substantially or because tooheavy an ionic bombardment has sputtered and thrown away a portion ofthese pieces. Experience has shown indeed that the metal thus depositedwould absorb mercury and gases during the operation of the lamp, whichwould prematurely put the lamp out of use.

One cannot provide the lamp with substantially more mercury than usual,in order to compensate for the eifect of a large surface of rare earthmetal, since the mercury droplets would damage too much the fluorescentlayer when the lamp will be handled. One cannot either provide the lampwith an excess of gas since the starting and operating voltages would besubstantially raised, the more as a large excess would be necessary.

It is advantageous, from this point of view, that the dischargeatmosphere of the lamp provided with an electrode according to theinvention comprise a noticeable proportion of nitrogen when the lamp isput into use, preferably at least 0.1 percent by volume. Nitrogen indeedlessens very substantially the sputtering of the electrodes by the ionicbombardment. As a matter of fact nitrogen will be absorbed during thenormal operation of the lamp but this absorption is very slow since therare earth metal olfers a small area only and the elfect of nitrogenwill be felt during a long duration.

Figure 2 shows the variation, as a function of the intensity of thedischarge current, of the voltage drop at the two electrodes of tubescontaining, in addition to a drop of mercury, a mixture of 80% argon and20% neon, under a pressure of 6 mm. mercury. This argon contains about0.3% nitrogen by volume. The curve 10 is relative to a pair of coldelectrodes not activated, each one of them being constituted asrepresented in Figure 1, except that it does not comprise any piece '6and being 60 mm. long and 12 mm. in diameter. These electrodes are usedindustrially for currents from 50 to 100 milliamperes. The curve 11 isrelative to a-pair of electrodes similar to the above but provided withfour segments6 of lanthanum wire, mm. long and 1.2 mm. in diameter. Thepoints used for plotting these curves were calculated from measuredresults obtained on tubes of difierent lengths so as to find out whatportion of the voltage drop in the tube occurs at the electrodes. Theaxis for abscissae is calibrated in milliamperes, that for ordinates involts.

The study of non activated electrodes was carried out only to 125milliamperes, as, beyond this value, the electrode is destroyed by ionicbombardment. In contrast the' electrodes according to the invention,having the same dimensions, withstand very well a 250 milliamperescurrent. A cold and non activated electrode, designed for this lattercurrent, should have an area which is twice as large as that of theelectrodes on which the measurements were made. Since, for this type ofelectrodes, the ratio length to diameter should not exceed a certainvalue (depending, in particular, on the pressure and nature of thefilling gas) it would be necessary to increase the diameter of theelectrode, which would make it necessary to house it in an electrodechamber with a larger diameter than that of the remaining portion of thetube. This drawback is obviated by the electrodes according to theinvention. Conversely, with elecetrodes according to the invention,smaller dimensions may be adopted if they are to operate only up tomilliamperes.

An advantage of the electrodes according to the invention, an advantagewhich is visible immediately in Figure 2, is the decrease in the voltagedrop at the electrodes. In the example illustrated the electrodesaccording to the invention gain about 30 volts for a 50 milliamperescurrent and 70 volts for 100 milliamperes. This peculiarity is fairlysurprising, since the activating pieces 6 have only a small area and arenot raised to a high temperature.

Another advantage of these electrodes, due probably to'the absorption ofthe detrimental gases by the pieces 6 is the decrease in the voltagedrop through the discharge column. This decrease varies largelyaccording to the case considered.

The discharge, substantially, does not occur on the outer surface of theelectrode because of the presence of electron-emissive material inside.This stabilizes the cathode glow and decreases considerably the cathodesputtering at the outside of the electrode. This makes it often possibleto simplify the constitution of the electrode by omitting the insulatingparts such as those shown at 2 and 7, and by not surrounding theelectrode with a sheet of mica, a sheet often used for improving theinsulation between the electrode and the glass of the discharge tube.This decreased sputtering increases the life of the electrode.Conversely it is possible to preserve for the latter a normal life whiledecreasing the pressure of the permanent filling gas. This pressuredecrease is known to improve the efficiency of the discharge in luminousand ultra-violet radiations at the expense of the electrode life.

Numerous modifications may be made to the electrode described withoutdeparting from the scope of the present invention; for example, thepieces 6 may have other shapes and other dimensions than thoseillustrated. They may also vary widely in number and they may be securedat various places on the inner wall of the electrode. The sheet metalpart 1 may be closed by a metal cap instead of the ceramic part 7 and ifdesired the part 1 may in certain cases be conical.

What I claim is:

'l. A discharge device having a gas and mercury vapor atmosphere andprovided with a cold and hollow electrode comprising at least one smallmetal piece fixed to the inner wall of said electrode, said piececomprising at least mainly at least one rare earth metal in a metalliccondition, and the area of said piece being small and remaining small ascompared with the area of the inner wall of the electrode at leastduring almost the whole life of the electrode, said gas comprisingsolely rare gas and a small proportion of nitrogen, said proportionbeing greater than 0.1% by volume. I

2. A discharge device having a gas sealed therein'and provided with acold and hollow electrode comprising at least one small metal piecefixed to the inner wall of said electrode, said piece comprising atleast mainly at least one rare earth metal in a metallic condition, andthe area of said piece being small and remaining small as compared withthe area of the inner wall of the electrode at least during almost thewhole life of the electrode, said gas comprising solely rare gas and asmall proportion of nitrogen, said proportion being greater than 0.1% byvolume.

References Cited in the file of this patent UNITED STATES PATENTSMetcalf Apr. 5, 1932 Szigeti Oct. 10, 1933 Foulke July 10, 1934 SpannerJuly 15, 1941 Eknayan Mar. 16, 1943 Lockwood Sept. 21, 1948

1. A DISCHARGE DEVICE HAVING A GAS AND MERCURY VAPOR ATMOSPHERE AND PROVIDED WITH A COLD AND HOLLOW ELECTRODE COMPRISING AT LEAST ONE SMALL METAL PIECE FIXED TO THE INNER WALL OF SAID ELECTRODE, SAID PIECE COMPRISING AT LEAST MAINLY AT LEAST ONE RARE EARTH METAL IN A METALLIC CONDITION, AND THE AREA OF SAID PIECE BEING SMALL AND REMAINING SMALL AS COMPARED WITH THE AREA OF THE INNER WALL OF THE ELECTRODE AT LEAST DURING ALMOST THE WHOLE LIFE OF THE ELECTRODE, SAID GAS COMPRISING SOLELY RARE GAS AND A SMALL PROPORTION OF NITROGEN, SAID PROPORTION BEING GREATER THAN 0.1% BY VOLUME. 